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Not many direct substitues exist for the 12AX7. It doesnt help that large numbers of users (and even technicians) are confused about the proper application of preamp tubes.

Although the "9A" base pinout is as close the industry ever came to a standard, many people have plugged in a tube with a totally different pinout or function, resulting in damage to the amp or preamp.

 

The 5751 (mu 70).

Today some audio tube users have found that 5751s can be used in place of 12AX7s in preamps, giving lower voltage gain yet much improved distortion and microphonics performance. The 5751s heater draws slightly more current than that in a 12AX7, 175mA versus 150 mA.

 

The 12AD7

Supposed to be a low-noise, low-heater-hum-induction, low-mircophony version of the 12AX7, predating the 7025.

Not popular in America or Europe, apparently was widely used in Japan.

12AD7 appears to be electrically identical to the original 12AX7, with one exception, the heater draws 12.6 volts at 225 mA.

12AD7s are quite scarce today.

 

12AT7

Often seen in guitar amps as phase inverter or reverb driver.

Many experts recommended against its use in hi-fi due to poorer linearity.

 

There are a few obsure types that are similar to the 12AX7. These includes the 12BZ7, 12DF7, 12DM7, 12DT7, 6681, 7729, Europ/Chinese 6L13, B339, B759, CV492, CV4004, CV8156, CV8222, E83CC, E2164, ECC863 and M8137.

 

This is an extract from VTV, hope its interesting to tubes lovers. :)

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